Michael Gelfond aaai:Gelfond91,amai:Gelfond94 recognized epistemic
negation to be a useful construct for ASP early on and proceeded to introduce
the modal operators \(\mathbf {K}\)  (“known” or “provably true”) and
\(\mathbf {M}\)  (“possible” or “not provably false”) to add this feature to the
language. \(\mathbf {K}a\)  and \(\mathbf {M}a\)  correspond to \(\lnot \mathbf {not}\,a\)  and
\(\mathbf {not}\,\lnot a\) , respectively. Renewed interest in recent years has revealed
several flaws in the original semantics, and various new approaches (cf. e.g. [1]}, [2]}, [3]}, [4]}, [5]}) were proposed. Also, several efficient and practical ELP solving
systems have been and continue to be developed [6]}, [7]}, [8]}.
